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Initially published in The Connecticut Warbler, July 1998, volume 18, number 3, p.120-32.

©1998 Connecticut Orinthological Society


Nest success rates and rates of fish delivery to nests were determined for two large Osprey populations in COlmecticut, one at Groton Reservoir, Groton, and one at Great Island, Old Lyme, during 1996 and 1997. Between 1993 and 1996 these Osprey populations had substantially different rates of nest success. Great Island Ospreys fledged few young while Groton Reservoir Ospreys had good nest success. During 1997, however, fledging rates were similar at the two sites. In 1996, low nest success at Great Island 'resuited from high predation rates, probably due to raccoons. The higher nest success rate at this site in 1997 appears to be due to low predation rates because of the installation of new predator guards on all nest platforms. There was no evidence of raccoon predation at Groton Reservoir in either year. In 1996 Ospreys delivered Hsh to their nests at a similar rate on Great Island and Groton Reservoir. In 1997 Great Island Ospreys made more Hsh deliveries to their nests than Groton Reservoir Ospreys.

The Osprey (Pandion haliaetus) became a symbol of the environmental movement during the 1960's and 1970's. Several studies at that time showed that this once abundant fish hawk had declined radically in numbers. In the Connecticut River estuary and surrounding areas, more than 200 active Osprey nests were documented in 1940, but by 1970 only eight active nests remained (Spitzer 1980, as cited in Poole 1989). The Osprey's decline was eventually traced to the chemical DDT and other organochlorines that were commonly used as pesticides and routinely sprayed on marshes to control mosquitoes during the 1940's and 1950's (Ames 1966). Since the use of DDT and other insecticides was balmed in the 1970's, Osprey numbers along the East Coast have been increasing (Spitzer et a!. 1978).

In some areas, such as the Delaware Bay and the Chesapeake Bay, Osprey populations are not making a full recovery. Predation by the Great Horned Owl (Bubo virginian us) was ad vanced as the most likely cause for nest failures along the New Jersey side of the Delaware Bay (Steidler et a!. 1991), while food shortage and sibling aggression were deemed the reason for nest failures in the Chesapeake Bay (McLean and Byrd 1991). Sibling aggression and brood reduction, which were common in Chesapeake Bay Ospreys, are often caused by food shortage, but are rarely observed in Ospreys with no food stress (O'Conner 1978; Stinson 1979; Poole 1984). Also, during the nesting months the male Osprey does 99.9% of the hunting for his family (Poole 1989). Food stress could be an indired caU$e of the nest failure of Ospreys; if the male camlOt find enough food, the female may be forced to hunt also, thereby leaving eggs or chicks vulnerable to predation.

There were 106 active Osprey nests (nests with eggs) in Connecticut during 1996 and 131 active nests during 1997 (Victoria 1996, 1997). The last stronghold for nesting Ospreys in Connecticut during the DDT years was Great Island, a salt marsh located in Old Lyme. Historically, the densest colony of Ospreys in COlmecticut has been located in this marsh, and the nests at this site have had a high success rate. DUring the late 1970's and into the 1980's, when Ospreys began to recover from organochiorine poisoning, Great Island Ospreys continued to increase in numbers and to reproduce effectively. Since 1991, however, the number of successful nests on Great Island had declined dramatically, falling to zero in 1993 and remaining low through 1996 (Victoria 1996; Figure 1). The number of nesting Osprey pairs on Great Island remained high, but their nests produced few fledglings.

Groton Reservoir in Groton, Connecticut has a similar number of nesting pairs to the Great Island population. The Groton Reservoir Ospreys have been very successful at producing young during the same time period that the Great Island Ospreys were reproducing poorly (Figure 1). Great Island Ospreys Hsh mostly on Long lslalld Soulld and surrounding brackish estuaries, while the Groton Ospreys appear to be Hshing mainly at the freshwater reservoir where they nest (personal observation).

New England Ospreys liVing along the coast apparently rely mainly on three species of fish; winter flounder (P/euronecles americanus) make up 50% of the bird's diet, Atlantic menhaden (Brevoortia tyrannlls) account for 20% and river herring (A/osa spp,) account for 20% (Poole 1989). Survey trawls by the Connecticut Departulent of Environulental Protection, Fisheries Division, showed that the abundance of winter flounder off the Connecticut coast in 1995 was the lowest since data collection began in 1979 (Simpson et a!. 1995; Figure 2). The decrease in winter flounder coincided closely with the initial decrease in llest success at Great lsland. If the prey species that makes up 50% of the Osprey diet was less available to Ospreys at Great Island, this might affect nest success. Moreover, the Osprey population at Great Island is large, so there might be many males trying to find enough food for their families in approxinlately the same area. In contrast, the Groton Reservoir Ospreys apparently feed mainly on freshwater fish, so they would not be affected by the decline in fish populations in Long Island Sound.

This study focused on comparing the diets of Ospreys at Great Island 'and Groton Reservoir. The main goal was to determine whether the amounts of fish delivered to nests at the two sites were similar. Also, observations on any human and animal activity that might affect the Ospreys were recorded to attempt to determine whether there are other reasons that Great Island Ospreys were unsuccessful at breeding while Groton Reservoir Ospreys were successful.



The views expressed in this paper are solely those of the author.